The present invention relates to apparatus and methods for dispersing and feeding dry powders, more specifically, to an apparatus and a method of dispersing dry powders into an airstream of particles without using pressurized air, and feeding dry powders for particle analysis with a controlled rate.
Dry powders composed of small particles are used and produced in many different industries. Examples of such powders include food, pharmaceuticals, cements, abrasives, pigments, toners, and surface coating materials. It is often very important to measure and control the size of these dry powders as the size can impact the function of a material. An example is the size of pharmaceutical powders which affects the rate at which the drugs are dissolved and absorbed into the body.
There are many methods available for determining the size of particles, including laser diffraction, image analysis and time-of-flight, among others. When measuring the size of dry powders using these methods, problems are often encountered with agglomeration of individual particles into clusters of particles, which results in an incorrect measurement of the particle size. This agglomeration of particles, mainly due to Van der Waals interaction and hydroscopic forces, and various methods to de-agglomerate these clusters are well known in the prior art.
Ideally, a dry powder dispersion and feeding system should create a region of high shear forces, which disperse particle clusters. It can further create wall impacts and particle to particle impacts high enough to disperse clusters of particles, but low enough to prevent xe2x80x9cmillingxe2x80x9d, a phenomenon of the individual particle fracturing into smaller particles. For particle sizing measurement, it is also desired to provide the entire sample for a measurement, and to provide the sample at the rate required by the measurement instrument. It is also desirable to restrict the sample powder in the system and to collect the material at the end of the measurement to prevent contamination of the instrument or laboratory environment by potentially hazardous powders.
U.S. Pat. No. 4,573,801 (to Leschonski et al) discloses a method and a dispersing apparatus for producing a gas-solid two phase jet having a constant flow rate and predetermined velocity, therein the solids being fully dispersed. The apparatus has a complex structure comprising a metering groove, a vibrating feeder chute, wiper means for removal of surplus solids, means for compressing the remaining solids, and a flow channel including an injector. In the flow channel the solid particles are accelerated and fully dispersed by the shear force, and the resulting gas-solid particle mixture is discharged as a free jet. Furthermore, before discharge the gas-solid particle mixture is directed several times against impact surfaces to obtain full dispersion. This apparatus uses a groove filling mechanism which is not able to measure the entire sample presented to the instrument.
U.S. Pat. No. 4,895,034 (to Poole) discloses powder dispersing methods and apparatus for use in an aerodynamic particle sizing system utilizing a time-of-flight measurement technique. The apparatus includes a sample container, a device for agitating the powder sample, a separating device, and an air flow for carrying the particles to the outlet. The separating device includes a small annular orifice in which air flow applies high shear forces to clusters of particles to separate them.
U.S. Pat. No. 5,522,555 (to Poole) discloses a dry powder dispersion system. The powder disperser includes a dynamic shear dispersion assembly, and a fluidization assembly, connected by a momentum tube. A pulsed gas jet directed a powder sample in a chamber disperses particles of the powder sample, and the particles are transported into the dynamic shear dispersion assembly, where the particles are accelerated from an impact surface in a rapid acceleration chamber through an annular nozzle. The system controls the dynamic shear force applied to the aerosol transport gas, which affects reaction forces upon the particles within the nozzle.
U.S. Pat. No. 5,636,921 (to Murata et al) discloses a powder dispersing apparatus with a movable powder storing member. The apparatus comprises a pressure vessel, a suction nozzle, a scraper, and a powder storing member inside the pressure vessel, means for rotating and vertically moving the powder storing member. With this apparatus, a flow of air is directed near the sample surface in the powder storing member, and the scraper blade feeds the dry powder sample into the air stream. This feeding mechanism creates an additional surface which must be cleaned between sample measurements to prevent sample and system contamination.
All preceding prior art utilize pressurized air for dispersing dry powders, which brings in several disadvantages. Using pressurized air, in addition to vacuum required in particle sizing instruments, adds cost and complexity to the apparatus. Further, placing the dry powder under pressure raises safety concerns to the operators and working environment. In the event of a failure of the system, dry powders could be xe2x80x9csprayedxe2x80x9d into the instrument or the laboratory with potentially damaging results. An example is a pharmaceutical drug being sprayed into the laboratory environment causing injury or illness to the instrument operator. Moreover, It is known in the industry that the jet spraying of dry powders against surfaces causes particle milling, fracturing of the individual particles into smaller particles, when the jet velocities are high. Manufacturers of the particle sizing equipment which utilize the jet spraying methods often instruct the operators to determine the pressure levels where millng of the dry powders begins and maintain pressures below that pressure point. This creates an additional step in the method development and casts doubt on the measured results.
Therefore, it is apparent there exists a special need for a structurally simple, less expensive and environmentally safer dry powder dispensing and feeding apparatus, which meets industrial requirements for particle analysis.
In one aspect, the present invention relates to a dry powder disperser. The dry powder disperser comprises (a) a sample holder with a closed longitudinal wall, a open top and a closed bottom, (b) a suction probe consisting essentially of (1) an exterior wall with a like shape cross section to a cross section of the sample holder and with a smaller cross section dimension than cross section dimension of the sample holder, and (2) an internal suction channel with an orifice at a bottom of the suction probe, and extended through a top of the exterior wall of the suction probe, and (c) a vacuum means for providing a vacuum in the internal suction channel of the suction probe. When the suction probe is positioned above dry powders in the sample holder, and a vacuum is applied to the internal suction channel, non-pressurized ambient air enters a space between the exterior wall of the suction probe and the longitudinal wall of the sample holder, and forms an high velocity downward air flow along the longitudinal wall of the sample holder, and the downward air flow changing direction at the orifice of the suction probe and generating a high shear force above a surface of the dry powders. The high shear force disperses dry powders, and feeds dispersed dry powders into the internal suction channel.
The dry powder disperser has a ratio between a cross sectional area of the orifice and a cross sectional area of the space between the exterior wall of the suction probe and the longitudinal wall of the sample holder in a range from about 0.9 to about 1.5. Furthermore, the exterior wall of the suction probe is tapered at the lower end of the suction probe. An additional space formed by the tapered lower end of the suction probe above the surface of the dry powders creates air vortices, the vortices further generating a particle to wall and particle to particle impacts.
In another aspect, the present invention relates to an apparatus for dispersing and feeding dry powders. The apparatus comprises the dry powder disperser of the present invention comprising a sample holder, a suction probe, and a vacuum means; and a positioning means for positioning the suction probe in the sample holder.
The positioning means comprises a sample holder base supported by a lifting table, a lead screw for moving the lifting table longitudinally, a suspension arm for supporting the suction probe, and a control system. The positioning means positions the sample holder longitudinally to control a distance between the orifice of the suction probe and the surface of the dry powders inside the sample holder. Furthermore, the positioning means controls a feeding rate of dry powders into the internal suction channel by controlling a distance between the orifice and the surface of the dry powders in the sample.
In a further aspect, the present invention relates to a method of dispersing dry powders. The method comprises the steps of placing a dry powder sample in a sample holder comprising a closed longitudinal wall, a open top and a closed bottom; positioning a suction probe above the dry powder sample, wherein the suction probe comprises an exterior wall with a like shape cross section to a cross section of the sample holder and with a smaller cross section dimension than cross section dimension of the sample holder, an internal suction channel with an orifice at a bottom of the suction probe, extended through a top of the exterior wall of the suction probe, and the suction probe being connected to a vacuum means for providing a vacuum in the internal suction channel; and applying vacuum to the suction probe, wherein non-pressurized ambient air enters a space between the exterior wall of the suction probe and the longitudinal wall of the sample holder, and forms an high velocity downward air flow along the longitudinal wall of the sample holder, and the downward air flow changing direction at the orifice of the suction probe and generating a high shear force above a surface of the dry powders; and wherein the high shear force disperses dry powders, and feeds dispersed dry powders into the internal suction channel.